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          <h1 class="post-title" itemprop="name headline">为什么你的广播包收不到</h1>
        

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        <p>‘为什么收不到广播包’，听到的答案都是被路由丢弃了，其实这只是一种情况，而这种情况是比较好理解，一般都会规避，往往实际情况不是这样，比如：</p>
<ul>
<li>同一个路由器下，两台设备的广播包到不了</li>
<li>两台设备网线直连，广播包到不了</li>
</ul>
<p>本文将为大家一一介绍其中的缘由.</p>
<h3 id="1-概念"><a href="#1-概念" class="headerlink" title="1. 概念"></a>1. 概念</h3><p>广播包，即发给所有本地主机的包，分为：</p>
<ul>
<li>受限广播包（255.255.255.255）- 不被转发</li>
<li>子网广播包（netid.1）- 往所有子网广播</li>
<li>主机广播包（netid.subnetid.1）- 往指定子网广播</li>
</ul>
<p>这里有一点需要强调：广播包是基于 UDP ，TCP 是没有广播包的说法</p>
<h3 id="2-用途"><a href="#2-用途" class="headerlink" title="2. 用途"></a>2. 用途</h3><p>广播包的用途自然就不用说，ARP、DHCP等一些非常重要的协议都是基于广播包。当然也不能滥用，否则会影响网络及主机性能，就好比你天天都在小区大喊，谁也受不了。</p>
<h3 id="3-阻碍因子"><a href="#3-阻碍因子" class="headerlink" title="3. 阻碍因子"></a>3. 阻碍因子</h3><p>广播包既然可以发给所有主机，按理说所有主机都需要处理它，那有哪些因素会阻碍『收广播包』？</p>
<h4 id="3-1-路由器"><a href="#3-1-路由器" class="headerlink" title="3.1. 路由器"></a>3.1. 路由器</h4><p>为了保证网络的干净，路由器一般不转发广播包，于是这里大家常常误解，以为只要广播包一接入路由器，都有可能被路由器丢弃；于是往往有个答案『你的广播包被路由器丢弃』，其实只有经过 WAN口的广播包才会丢弃。因此如果是2个 LAN 口间的广播包是不会丢弃的，即使两个主机 IP 不一样。</p>
<p>比如：192.168.1.88与192.168.2.88两台主机连到192.168.2.1路由上照样可以正常收发广播包。</p>
<h4 id="3-2-多网卡（发送方）"><a href="#3-2-多网卡（发送方）" class="headerlink" title="3.2. 多网卡（发送方）"></a>3.2. 多网卡（发送方）</h4><p>默认是不会往所有网卡发送广播包，广播包跟其他数据包一样，也是通过本地路由表来决策该往哪个网卡发送，比如有一台双网卡，分别是61和161网段，有如下路由表：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">Kernel IP routing table</span><br><span class="line">Destination     Gateway         Genmask         Flags Metric Ref    Use Iface</span><br><span class="line">...</span><br><span class="line">0.0.0.0         192.168.161.1   0.0.0.0         UG    0      0        0 em1</span><br><span class="line">0.0.0.0         192.168.61.1    0.0.0.0         UG    0      0        0 em2</span><br></pre></td></tr></table></figure>
<p>广播包是往 em1发送，而不会往 em0发送（优先匹配前面规则），当然你也可以指定往哪个网卡发送</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">srcaddr.sin_addr.s_addr=inet_addr(&quot;192.168.61.223&quot;);</span><br></pre></td></tr></table></figure>
<p>实测 window 系统，也是如此.</p>
<p>因此，如果是双网卡，请确保是否发对网卡。</p>
<h4 id="3-3-系统内核过滤（接收方）"><a href="#3-3-系统内核过滤（接收方）" class="headerlink" title="3.3. 系统内核过滤（接收方）"></a>3.3. 系统内核过滤（接收方）</h4><p>还有一种情况，比如 linux 系统增加了一种 Reverse Path Filter，即会对来源 IP 合法性进行判断，即是否本该从该网卡发过来，若不符合，则丢弃。简单地判断就是验证是否可以往相应网口向该 IP 发送数据。</p>
<p>比如你的路由表如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">Kernel IP routing table</span><br><span class="line">Destination     Gateway         Genmask         Flags Metric Ref    Use Iface</span><br><span class="line">192.168.161.0   192.168.161.1   255.255.255.0   UG    0      0        0 em1</span><br><span class="line">192.168.161.0   0.0.0.0         255.255.255.0   U     0      0        0 em1</span><br><span class="line">192.168.61.0    0.0.0.0         255.255.255.0   U     0      0        0 em2</span><br></pre></td></tr></table></figure>
<p>则以下情况都是不合法来源</p>
<ul>
<li>在网口 em1，收到了192.168.61.xx的数据包（应该在 em2上收到）</li>
<li>网卡 em1或 em2收到10.10.xx.xx的数据包</li>
</ul>
<p>该机制是为防止恶意攻击，具体各位可以去了解下。那么是否可以关闭该特性，肯定是可以.</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">vim /etc/sysctl.conf</span><br><span class="line"></span><br><span class="line">net.ipv4.conf.default.rp_filter = 0</span><br></pre></td></tr></table></figure>
<p>即可，当然也可以增加路由表来解决。</p>
<p>该特性只在 linux 上，window系统就不得而知了。</p>
<h3 id="3-4-VLAN"><a href="#3-4-VLAN" class="headerlink" title="3.4 VLAN"></a>3.4 VLAN</h3><p>为了防止大量广播消耗网络资源，有些大型局域网会使用VLAN技术，将交换机下的某些口组成一个单独的 LAN，这样广播包只会在该 LAN，而不会发到其他端口上.</p>
<h3 id="4-总结"><a href="#4-总结" class="headerlink" title="4. 总结"></a>4. 总结</h3><p>当你遇到广播包收不到问题，可以基于如下思路排查：</p>
<ul>
<li>确认物理连接是否属于统一网段(注意是物理连接，而不是说IP)</li>
<li>确认发送发是否发对网卡</li>
<li>如果是接收方是 linux，确认系统是否开启 rpfilter</li>
</ul>

      
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